The Agriculture Department announced the discovery of the modified wheat on Wednesday. No genetically engineered wheat has been approved for U.S. farming.

Japan is one of the largest export markets for U.S. wheat growers. Katsuhiro Saka, a counselor at the Japanese Embassy in Washington, said Thursday that Japan had canceled orders of western white wheat from the Pacific Northwest and also of some feed-grade wheat. He said the country was waiting for more information from the Agriculture Department as it investigates the discovery.

"In most countries the unapproved genetically modified wheat would be a target of concern," Saka said. "The Japanese people have similar kinds of concerns."

USDA officials said the wheat was the same strain as a genetically modified wheat that was designed to be herbicide-resistant and was legally tested by seed giant Monsanto a decade ago but never approved. Monsanto stopped testing that product in Oregon and several other states in 2005.

The Agriculture Department said the genetically engineered wheat is safe to eat and there is no evidence that modified wheat entered the marketplace. But the department is investigating how it ended up in the field, whether there was any criminal wrongdoing and whether its growth is widespread.

The mystery could have implications on the wheat trade in the U.S. and abroad, as evidenced by Japan's suspension of imports on Thursday.

Many countries around the world will not accept imports of genetically modified foods, and the United States exports about half of its wheat crop.

U.S. consumers also have shown increasing interest in avoiding genetically modified foods. There has been little evidence to show that foods grown from engineered seeds are less safe than their conventional counterparts, but several state legislatures are considering bills that would require them to be labeled so consumers know what they are eating.

While most of the corn and soybeans grown in the United States are already modified, the country's wheat crop is not. Many wheat farmers have shown reluctance to use genetically engineered seeds since their product is usually consumed directly, while much of the corn and soybean crop is used as feed.

The modified wheat was discovered when field workers at an Eastern Oregon wheat farm were clearing acres for the bare offseason when they came across a patch of wheat that didn't belong. The workers sprayed it and sprayed it, but the wheat wouldn't die. Their confused boss grabbed a few stalks and sent it to a university lab in early May.

A few weeks later, Oregon State wheat scientists made a startling discovery: The wheat was genetically modified, in clear violation of U.S. law. They contacted the USDA, which ran more tests and confirmed their discovery.

The tests confirmed that the plants were a strain developed by Monsanto to resist its Roundup Ready herbicides and were tested between 1998 and 2005. At the time Monsanto had applied to the USDA for permission to develop the engineered wheat, but the company later withdrew its application.

Wheat is safe to eat, feds say

The Agriculture Department said that during that seven-year period, it authorized more than 100 field tests with the same glyphosate-resistant wheat variety. Tests were conducted in in Arizona, California, Colorado, Florida, Hawaii, Idaho, Illinois, Kansas, Minnesota, Montana, Nebraska, North Dakota, Oregon, South Dakota, Washington and Wyoming.

During that testing and application process, the Food and Drug Administration reviewed the variety found in Oregon and said it was as safe as conventional varieties of wheat.

USDA officials declined to speculate whether the modified seeds blew into the field from a testing site or whether they were somehow planted or taken there, and they would not identify the farmer or the farm's location. They said they had not received any other reports of discoveries of modified wheat.

Japan is regularly the top buyer of Northwest wheat, said Blake Rowe, CEO of the Oregon Wheat Commission. He said reductions in wheat sales would affect farmers in Idaho and Washington as well as Oregon, because the wheat is blended together.

Oregon sold $492 million in wheat in 2011, the most recent data available, and 90 percent of it went overseas, Oregon Department of Agriculture spokesman Bruce Pokarney said.

"If those markets are closed off — you can do the math," Pokarney said.